Passive intermodulation (PIM) is an unwanted signal generated by the mixing of two or more frequencies in a passive device such as a connector or cable. In general, PIM signals are created when two signals combine and are reflected at a point such as a faulty cable connector. PIM is an issue in cellular telephone technologies in which, for example, cable assemblies connecting a base station to an antenna on a tower have multiple connectors that may cause PIM that can interfere with system operation ultimately affecting quality of service.
Products of Intermodulation occur in active (IM) or passive devices (PIM) when two or more signals mix altogether. In passive devices, these unwanted mixes are created by non-linearities that may be caused for example by dirty surfaces, loose connections, poor soldering, etc.
In radio communication networks, PIM signals are to be avoided as they may interfere with signals within reception frequency bands, reducing for instance the data rate of communication budget links. Today's advanced architectures for transmission and reception makes the communications systems more vulnerable to interference.
As the causes of PIM failure can be numerous, localization of PIM root causes can be long, complex, and require experimented technicians—and so are costly—especially when the causes occur within an RF device comprising numerous components as sub-modules.
PIM can be reduced by replacing or repairing PIM root causes. Test systems can be utilized to detect PIM, thereby enabling location of the faulty parts. For example, test systems can generate two signals at two different frequencies, amplify them, and provide them to a device under test. A return signal is filtered to select a desired test frequency harmonic where PIM can be detected, and the distance to the possible cause of the PIM measurement is provided to an operator.
PIM sources can occur within a “static mode” or a “dynamic mode” as follows.
A “static mode” refers to where PIM source levels are continuous and stable over time.
Existing PIM measurement materials integrating Distance To Fault (DTF) features may be used to localize PIM. These existing PIM DTF measurement equipment operations are based on the measurement of the time difference between the transmitting test signal and the receiving incoming PIM product. Capabilities of PIM localization techniques are linked to the resolution (ability to discriminate two or more different PIM sources) and accuracy capabilities (absolute precision of localization of a single PIM source), which are mainly linked to the frequency bandwidth, the PIM root cause level and the noise floor of the PIM test bench, as the signal processing methodology used.
For instance, FIG. 1 is a diagram of a typical rooftop radio communication installation consisting of linked RF cables (comprising connectors), one Remote Radio Head (RRH) and one antenna. Typical PIM DTF measurement materials may have an accuracy of about 20 cm and a resolution of about 2 meters for PIM root causes having levels close to the 3GPP (Third Generation Partnership Project) specification which relates to a telecommunications standard, i.e. near −110 dBm when 2×43 dBm carriers are injected within the Device Under Test (DUT). Within this context, and taking account that the distance between the RF connections or between RF modules are greater than the PIM DTF measurement material resolution, the probable PIM root cause can be determined, i.e. localized between the RF connections as RF modules that sound faulty, due to their related “large” electrical distance between them.
If localizing at a global site which main element is faulty (the cable, the RRH, the antenna etc.), localizing the PIM root causes inside these elements (i.e. in the faulty RRH or in the faulty antenna) is much more difficult due to the circuitry complexity. The circuitry complexity is not simply an addition of materials placed in series and so requires much more precision. Further, several probable PIM root causes may be distances apart of only a few cm or even a few mm and so require much higher precision. For example, in a panel antenna as shown in FIG. 2, where there is a faulty radiating element among several radiating elements, since the electrical distance from the input to each of the radiating elements may be similar or the same (i.e. the time difference for a test signal from the “input” to each of the radiating elements will be similar or the same), it will be difficult to determine which of the several radiating elements is the faulty radiating element.
A “dynamic mode” refers to where PIM source levels are not continuous and/or are not stable over time.
The complexity of PIM sources localization is largely increased if PIM source levels are not stable over time, for example a random or dynamic mode, or, are linked to potential stresses applied to the DUT such as temperature variations, vibrations, shocks, etc. In this case, the measurements captured by existing DTF PIM equipment have major drawbacks as the PIM levels can significantly vary during the capture/data acquisition stage. As a consequence, performances of existing standard DTF PIM equipment are greatly reduced and can even determine incorrect locations of PIM source among other major dysfunctions.
Therefore existing PIM DTF methods are not sufficient within either static or dynamic modes. The best known commercialized PIM DTF measurement materials cannot be used to localize PIM within complex RF system context. Best known performances in resolution and accuracy—in the range of 2 m and 20 cm respectively—limit the possibility to localize a faulty subcomponent and to detect multiple PIM sources in a complex system such as an antenna.
Hence, a new PIM DTF method that permits to localize PIM within either a static and/or a dynamic mode and which deeply enhances resolution and accuracy precision capabilities is required. There is therefore a clear need of PIM DTF equipment having much better capabilities those currently available. For instance, around the 2 GHz band, such equipment would need to have a resolution of about 20 cm and an accuracy of about 2 cm to permit to point out faulty PIM sources, i.e. internal connections as subcomponents.